[Variant] Fix NULL handling for shredded object fields

[scovich]

Which issue does this PR close?

• Fast-follow for [Variant] Define new shred_variant function #8366
• Related to Add Arrow Variant Extension Type, remove Array impl for VariantArray and ShreddedVariantFieldArray #8392

Rationale for this change

Somehow, #8392 exposes a latent bug in #8366, which has improper NULL handling for shredded object fields. The shredding PR originally attempted to handle this case, but somehow the test did not trigger the bug and so the (admittedly incomplete) code was removed. See #8366 (comment). To be honest, I have no idea how the original ever worked correctly, nor why the new PR is able to expose the problem.

What changes are included in this PR?

When used as a top-level builder, VariantToShreddedVariantRowBuilder::append_null must append NULL to its own NullBufferBuilder; but when used as a shredded object field builder, it must append non-NULL. Plumb a new top_level parameter through the various functions and into the two sub-builders it relies on, so they can implement the correct semantics.

Are these changes tested?

In theory, yes (I don't know how the object shredding test ever passed). And it fixes the breakage in #8392.

Are there any user-facing changes?

No

[alamb]

Thanks @scovich

[scovich]

@alamb -- we should track a follow-on item somewhere: Once we support array element shredding, we will have three different semantics to deal with, and this fix will need to expand from a bool to an enum:

When appending NULL:

	struct	value	typed_value
Variant	NULL	NULL	NULL
Object field	valid	NULL	NULL
Array element	valid	Variant::Null	NULL

[alamb]

@alamb -- we should track a follow-on item somewhere: Once we support array element shredding, we will have three different semantics to deal with, and this fix will need to expand from a bool to an enum:

Filed

• [Variant] extend shredded null handling for arrays #8400